Developmental language disorders affect at least 5-7% of the US population, and are particularly hard to diagnose in English Language Learners (ELL), putting them at risk for under- diagnosis and loss of appropriate services. Spanish speakers account for the vast majority of ELLs in the US, yet basic research on the language development of Latino children learning Spanish has been limited, and little is known about potential risk markers for language impairment (Ll) in this population. Existing work on the morphosyntactic manifestations of Ll in Spanish speakers is based almost exclusively on measures of expressive ability, with next to no evidence on online and offline receptive abilities in this domain. This research has two specific aims: 1) to determine how knowledge and use of two morphosyntactic properties previously identified as particularly vulnerable in Spanish Ll - object clitics and articles - emerge during the 4th year of life in Spanish-learning children distributed across the language endowment spectrum, a population underrepresented in the scientific literature;2) to evaluate current theoretical models of Ll, which make contrasting predictions on the differential impairment of (a) production vs. comprehension, (b) online vs. offline performance, and (c) linguistic vs. non-linguistic abilities. To these ends, Spanish-learning children (n=46), with special emphasis on representation at the lower end of the language endowment spectrum, will be tested at three time points (ages 3;0, 3;6, 4;0) on measures of online processing, offline comprehension, elicited production, and standardized assessments of cognitive ability and working memory (WM). Correlation analyses will examine the relations between children's skills with the SAME two morphemes in different areas of language use, and their relation to standardized measures. Growth curve analyses (HLM) will be used to track developmental change over time. Between-group analyses of children with 'weak'vs. more 'solid'language abilities on the tightly-controlled three-fold assessment of production, comprehension and processing of these two morphemes, as well as WM, will allow the precise evaluation of several key predictions of current models of language development and Ll, thus making a direct theoretical contribution. RELEVANCE: Spanish-learning children in the US are at risk for underdiagnosis of Ll. Valid and reliable diagnostic tools for the identification of Ll in this population are lacking, reflecting limitations in basic research on the manifestations of Ll in Spanish more generally. The proposed research will provide the first evidence regarding the early development of on- and offline receptive abilities in morphosyntax, thus filling a gap in basic research which may lead to the development of better tools for early diagnosis and intervention.